The present invention relates to a method of directly and continuously determining the magnitude of the earth's gravity at one or more selected points and, more particularly, it relates to a method for directly and continuously determining the earth's gravity at one or more selected points by way of a computerized arrangement for deriving the difference between the vertical accelerations of a Global Positioning System (GPS) user platform and the gravimeter acceleration of the same platform.
Various techniques have been developed by the prior art for determining the effects of the earth gravitational field along or above the earth's surface. These techniques have been important for navigational purposes; seismic studies, geological studies, etc. Such techniques have included rotating masses, which are counterbalanced, that respond to changes in a gravitational field so as to indicate the gravitational gradient at a preselected point Other techniques have included satellites arrangements for determining changes in the gravitational field at a particular point by radar altimetry and the Doppler effect. These prior techniques were often time-consuming and often did not exhibit the best accuracy for navigation purposes, etc. For example, U.S. Pat. No. 3,803,916 to DeMatteo et al. concerns the combined arrangement of a ship inertial navigation system (SINS) and a long range navigation signal from a ground station (LORAN) for determining the vertical deflection gravity at a selected point on the ocean surface. U.S. Pat. No. 4,295,372 to DeMatteo merely concerns an improved SINS. To this end the SINS is generally made up of a velocity meter and a computer with an inverse filter for indicating the gravity at a selected point on the ocean surface. Another U.S. Pat. No. 3,888,122 to Black discloses a double satellite system, with each satellite moving in the same orbital path about the earth, for measuring the earth gravitational field at a selected point on the earth surface. By reason of the double satellites having Doppler radar altimeters, the satellites are able to measure and indicate two independent components of the gravity at a selected point on the ocean surface. However, none of the aforediscussed patents, taken singly or in any combination, remotely suggest the use of a multiple earth-orbit, multiple satellite, and computerized Global Positioning System (GPS) for directly and continuously determining, not only in less time but with greater accuracy than heretofore obtainable, the gravity at one or more selected points on or above the earth (ground or ocean) surface. One advantage of the improved method of the computerized GPS system of the present invention is that more accurate determining of the gravity at one or more selected points enables the inertial navigation system of a vessel or aircraft to be corrected with greater accuracy. Further, obtaining the gravity data with greater accuracy at more than one selected point by virtue of the improved method of the invention is of assistance in numerically determining the angle of deflection between GPS determined vertical acceleration of a user platform and the gravimeter acceleration thereof. Such assistance is significant in accurately predicting the trajectory of a weapon or missile prior to firing or launch.